a shoulder on as size 4 blouse (the top shoulder seam) is only about 0.5inches different than on a size 24--(but a size 4 fits a 32 inches bust and a size 24 fits a 50inch one!)
this makes up sizing adult patterns difficult.--the increases need to be adusted.
but a new born baby has narrow, narrow shoulders, and then 6 months later, the shoulders are wider.
and bigger babies, (ie 8 lb, vs 4lb.) are bigger all over--you can just scale up.
use a thicker yarn (sports vs fingering) use larger needles (size 5 vs size 2) or just add stitches (go from 25 stitches to 35..)
when in comes time to do arm/necks.. keep the same ratio)
(25 sized up to 35 =20% increase.)
if the pattern calls for 10 (for neckline) then 10stitches +20%= 12-- will work for neckline.
(problems will arise with patterns.. (stitch pattern)
but it shouldn't be too hard (compared to up sizing adult clothes.